The present invention relates to a magnetic sensor for detecting the steering angle of a steering wheel.
Referring to FIG. 1, a prior art magnetic sensor 101 includes four magnetic resistors 111 to 114, which configure a Wheatstone bridge. When a preferred or high performance magnetic sensor is arranged in an electric field of a predetermined direction, the voltage difference of a node Q1, between the magnetic resistors 111 and 112, and a node Q2, between the magnetic resistors 113 and 114, is close to zero volts. The voltage difference is an offset voltage, which is the difference between median potentials E1 and E2 of the bridge. When the electric resistances of the magnetic resistors 111, 112, 113, and 114 are respectively represented by R11, R12, R13, and R14, the preferred or high performance magnetic sensor satisfies equation (1).R11×R14=R12×R13  (1)
FIG. 2 is an enlarged diagram showing the four magnetic resistors 111 to 114. Each of the magnetic resistors 111 to 114 is a thin film having a certain pattern and is formed on a substrate 110 of the magnetic sensor 101. The magnetic resistor 111 includes a magnetic detection portion 121, a rough adjustment portion 131, and a fine adjustment portion 141. The magnetic detection portion 121 is extremely narrow. The rough adjustment portion 131 is narrow and ladder-like. The fine adjustment portion 141 is extremely wide. The other magnetic resistors 112, 113, and 114 each have the same configuration as the magnetic resistor 111 and respectively include magnetic detection portions 122, 123, and 124, rough adjustment portions 132, 133, and 134, and fine adjustment portions 142, 143, and 144.
The procedures for manufacturing the prior art magnetic sensor 101 will now be described. In the magnetic sensor 101, the patterns of the magnetic resistors 111 to 114 are determined so as to equalize the electric resistances R11 to R14. When the electric resistances R11 to R14 are equal to one another, the offset voltage E12 is close to zero volts, and equation (1) is satisfied. However, the electric resistances R11 to R14 tend to differ from each other. Factors causing such differences include the formation accuracy of thin films and the etching accuracy for patterning the magnetic resistors 111 to 114. However, even if the thin film formation accuracy and etching accuracy for patterning the magnetic resistors 111 to 114 were to be improved, it would still be difficult to satisfy equation (1).
Japanese Laid-Open Patent Publication No. 05-034224 proposes laser trimming in which the magnetic resistors 111 to 114 are partially cut with a laser beam to finely adjust the electric resistances R11 to R14.
The laser trimming partially cuts one of the four magnetic resistors 111 to 114. This increases the resistance of the partially cut magnetic resistor and satisfies equation (1).
The laser trimming performed in the prior art will now be described with reference to FIG. 2. Laser trimming is performed to cut part of the rough adjustment portion 132 (trimming section 132b) and part of the fine adjustment portion 142 (trimming section 142b). The magnetic resistor 112 receives the heat generated by irradiation of the laser beam. The magnetic resistors 111, 113, and 114 do not receive the heat of the laser beam irradiation. Change in the electric resistances R11 and R14 subsequent to laser trimming differs between the magnetic resistor 112, which is irradiated with a laser beam, and the magnetic resistors 111, 113, and 114, which are not irradiated with a laser beam. Thus, even when performing the laser trimming to satisfy equation (1), the electric resistance R12 of the magnetic resistor 112 changes at a rate that differs from the rate at which the electric resistances R11, R13, and R14 of the magnetic resistors 111, 113, and 114 change as time elapses. Thus, the equilibrium of the Wheatstone bridge (equation 1) cannot be maintained as time elapses. Thus, the offset voltage E12 changes from zero volts, immediately after the laser trimming, to a level that cannot be ignored in regard with the capacity of the magnetic sensor 101 (refer to FIG. 3).